Nickel, cobalt, and nickel-cobalt plating have been widely used in industry for many years because of the uniformity of plating which can be obtained, regardless of the shape of the part being plated, and regardless of whether the part is made of metal or of plastic.
All the baths currently used for nickel and/or cobalt chemical plating, ie. plating by autocatalysis, regardless of whether the baths are acid or basic, use a reducing agent based on boron or on phosphorous in the form of an alkaline hypophosphite or a hydrogenated boron derivative. In addition to said reducing agent, such baths contain one or more salts of the plating metal(s), one or more complexing agents of said metal(s), and a stabilizing agent.
The complexing agent serves to keep the metal in solution and to liberate metal at the rate at which the plating reaction consumes it. The stabilizing agent is essential for an autocatalysed reaction such as the one used for chemical plating, and serves to reduce the speed of the plating reaction, thereby enabling the desired uniformity of plating to be obtained on the part being plated.
Industrially used stabilizing agents comprise organic and inorganic sulfur derivatives for use in acid baths, and for use in basic baths they comprise compounds of the metals and metalloids from groups IIIa, IVa and Va of the periodic classification of elements, and in particular thallium.
Although baths constituted in this manner are satisfactory for most common applications, the Assignee has encountered difficulties in their use for some specific applications.
The Assignee has been concerned, for example, with the manufacture of metal parts, such as turbine blades, which are required to operate at very high temperatures and in highly corrosive atmospheres, eg. containing sulfur. In this context a surface treatment process has been developed for protecting such parts by chromo-aluminization (see French Pat. No. 74 24694). This thermochemical process is performed on parts previously covered in nickel, and preferably by chemical nickel plating.
Unfortunately, it appears in use, that chemical plating using baths in which the stabilizing agent includes sulfur causes sulfur to be deposited together with the plating metal. The sulfur is deposited at a concentration of 3 to 5 parts per thousand by weight, and may lead to damaging corrosion phenomena in the plating and/or in the plated substrate. Similarly, the use of baths in which the stabilizing agent comprises a compound of a heavy metal such as thallium, leads to the heavy metal being present in the plating. When the heavy metal is at a concentration of about 5% by weight in the plating, it considerably reduces the mechanical properties of the plated alloy by diffusing and/or coalescing in the plating and/or the substrate.
Further, such chemical metal plating techniques are used for plating molds which are used in the manufacture of glass receptacles intended for storing food. Presently used stabilizing agents lead to the presence of toxic material which may diffuse into the glass and thus come into contact with a food product. The Assignee has thus found it desirable to be able to perform chemical nickel and/or cobalt plating using a bath which does not lead to the presence of toxic material in the layer of plating.
Published French patent application No. 2 329 762 proposes a type of stabilizing agent which differs from those usually used in that it is constituted by an organic iodine compound. Such compounds do not avoid the drawbacks of conventional stabilizing agents since the presence of iodine leads to comparable effects to the presence of sulfur. The same is true of other halogens.
Preferred embodiments of the present invention provide a bath for use in nickel and/or cobalt chemical plating, which bath includes a stabilizing agent that does not lead to the above drawbacks associated with prior stabilizing agents.